Mine, and this is worth posting because the cause was not on anybody's list.
Started at about age four. I went through everything — vet check, skin scrape, bloods, all clear. Bigger cage. More toys. Different diet. Nothing.
It was the air conditioner. We had put one in that room the previous summer and it ran ten hours a day. Grey parrots come from West African rainforest and they are built for humidity; a room at 30% humidity for months dries the skin, the skin itches, the bird chews. The vet who eventually worked it out asked one question nobody else had: "what changed in that room?"
What fixed it: a humidifier, and a proper shower twice a week — actual soaking wet, not a light mist. Within six weeks the chewing had almost stopped. The feathers took a full moult to come back and one patch on the thigh never did.
What I would say to anyone starting this:
Rule out medical first, properly, with a vet who sees birds — not a dog-and-cat vet having a guess. Mites, giardia, a skin infection, and liver disease all present as plucking and no amount of toys touches any of them.
Then look at what changed, not at what is wrong. Plucking usually starts, and it starts at a point in time. New AC. Someone moved out. New working hours. Cage moved to a quieter corner — which people do as a kindness and which a Grey can read as being excluded from the family.
And do not expect it to be quick. Anyone promising a fix in a fortnight is selling you something. Mine was months, and a bird that has plucked for years may never fully re-feather because the follicles give up. That is not a reason not to treat it — it is a reason to start now rather than after another year of hoping.
